A. About ammonia refrigerant
Ammonia is a good refrigerant, which belongs to the natural working fluid ODP=0; GWP=0, it has the best thermodynamic performance at present, the highest refrigeration efficiency, low price, low operation and maintenance costs, and it has a history of hundreds of years. The use of ammonia in large-scale refrigeration systems for domestic industrial and commercial refrigeration has reached more than 85%.
The vapor is colorless and has a strong irritating odor. A small amount of ammonia leakage (3 to 5 ppm) will be detected, and the explosive limit concentration in the air is 15 to 28%.
B. Hazardous source of ammonia
Operating room: manual labor, densely staffed, closed environment, emergency evacuation is difficult after a hazard occurs.
Cold storage: Food is piled up, the environment is closed, and it is not easy to find after leakage.
Computer room: A large amount of ammonia liquid is stored, and a key location where an explosion occurs again.
Nearby environment: Keep away from areas with high crowd density.
C. Seek truth from facts to evaluate ammonia
When some companies promote ammonia as a natural working fluid, they over-beautify the ammonia refrigerant. Ammonia is flammable, toxic, corrosive and other characteristics, which determine the need to strengthen the safety and technical measures of the ammonia system. Compared with the chemical, coal, petroleum, petrochemical and other industries, the risk of ammonia is relatively small. Daily life: electricity, water, Gas, buildings... Danger always exists. All accidents occur due to negligence, carelessness, and management loopholes. Man-made disasters are greater than natural disasters!
D. Reasons for disasters
1. Fire causes secondary disasters, fire, high temperature, pressure rise and explosion;
2. Improper operation of manual defrosting, resulting in liquid hammer or closed-circuit defrosting, causing pipeline bursts;
3. The frost accident of a single freezer caused a large number of casualties. Aging of equipment and pipelines or lax management;
4. System pipelines, valves and other facilities are aging, leaking, and design and installation are not standardized;
5. The use of materials that do not meet the fire protection standards, with ammonia for electric welding and other open flame operations.
E. Problems that cannot be avoided
1. The refrigeration system with direct ammonia evaporation has a large charge;
2. Ammonia directly enters the densely populated operation room and the food freezing and cold storage room;
3. In the event of force majeure such as fires and earthquakes, secondary disasters of ammonia will inevitably occur.
F. Ammonia safety program
1. Minimize the amount of ammonia charged. In GB18218-2009 "Identification of Major Hazardous Sources of Hazardous Chemicals", the critical amount of ammonia is 10t.
2. Limit ammonia to specific areas in the computer room, away from areas that can cause personal and food safety, such as operation rooms and cold storage
3. Ammonia system takes active defense measures, concentration monitoring and alarm prevention. Active defense: alarm, high-altitude discharge, spraying, equipment joint control and emergency stop.
G. Ammonia safety program
When an ammonia leak occurs in an area, the active safety defense system will depend on the amount of ammonia leaked:
1. The sound and light alarm reminds the operator that there is an ammonia leakage accident;
2. The emergency exhaust fan is activated to discharge the leaked ammonia to the outdoors;
3. Water curtain spraying will separate the hazardous area with water from the ammonia point.
If the ammonia leaks further, the IEMC joint control function will reduce the load and stop the compressor in the corresponding area
Technical points of water curtain spraying: prevent the explosion limit of ammonia formation, prevent water ingress from electrical switches, etc., shutdown control must be carried out in accordance with the refrigeration system process, avoid simultaneous shutdowns, and ensure the quality of equipment after the accident, spray water treatment system, spray The reliability and timeliness of the system.
H. Defrosting method: automatic instead of manual
Liquid inlet pipeline: stop valve→filter→solenoid valve→check valve→stop valve;
Air return pipeline: stop valve→two-stage open solenoid valve→stop valve;
Hot ammonia pipeline: stop valve→filter→solenoid valve→check valve→stop valve;
Discharge line: Bypass valve.
I. Liquid burst and liquid hammer
Liquid burst: Incompressibility of liquid, expansion by heat.
Precautions: Correct operation and avoid liquid hammer.
Causes of liquid explosion:
1. During the defrosting process, the liquid in the heat exchange tube group is full or there is too much liquid.
2. The valves at both ends of the cryogenic liquid pipeline are closed, and the pressure rises after heating (such as the valve from the ammonia pump to the regulating station.
Liquid hammer: high-speed liquid impact.
Prevention: Operate correctly, increase and decrease the pressure slowly.
The formation of liquid hammer is mainly in the following two situations:
1. Close the liquid supply valve before defrosting, and the ammonia liquid in the recovery heat exchange tube group is not cut to the bottom. There is liquid in the return gas pipeline. At the beginning of the defrost, the hot ammonia inlet valve opens too fast, and the high pressure gas pushes the return gas pipeline. Liquid, accelerated flow (pressure difference above 8bar affected by pressure difference) generates a high-speed liquid flow in the return air pipe, and liquid hammer will occur when it encounters obstacles. Be sure to slowly open the hot ammonia intake valve.
2. When the defrosting is completed, the liquid in the heat exchange tube group is not decompressed, and the return valve is opened too fast.
J. Increase warehouse emergency treatment facilities
1. Ammonia concentration detection and alarm device and water supply system.
2. Add shielding barriers to the liquid reservoir.
3. Alarm for super high liquid level in the liquid reservoir.
4. Specially set up high-altitude emergency ammonia to be discharged into the water container.
5. When the ammonia safety defense reforms the hot ammonia defrost, the pressure before entering the evaporator shall not exceed 0.8MPa.
6. It is forbidden to speed up the defrosting speed by closing or closing the condenser inlet valve.
7. The active defense measures of the ammonia system can be adopted in both the computer room and the warehouse.
8. Only by minimizing the filling volume can the harm be reduced.